Heavy Metal Stabilization Of Sludge-based Biochar And Its Adsorption Of Pollutants

The ecological risk of heavy metals has been restricting the resource utilization of sludge and sludge pyrolysis biochar,and the ecological risk of heavy metals largely depends on its chemical form.In recent years,the speciation distribution of heavy metals in sludge and its biochar has become a hot topic in the field of environmental research.At the same time,the adsorption effect of sludge-based biochar on typical pollutants in water phase needs to be further studied.This study aims to explore the effects of different pyrolysis temperatures,coconut shell and bentonite addition levels on the properties,chemical forms of heavy metals and ecological risks of sludge-based biochar,and further reveal the mechanism of heavy metal stabilization in sludge-based biochar.Finally,coconut shell-sludge-based biochar with the best stabilization effect of heavy metals was selected to study the adsorption of three typical pollutants in sewage.The results showed that the contents of six heavy metals in sludge were in the order of Zn > Cu > Cr > Ni > Cd > Pb,and the RI value of potential ecological risk of heavy metals in sludge was 397.78,indicating that direct exposure of sludge to the environment would cause higher potential environmental ecological risk.The increase of pyrolysis temperature(400-800 °C)and the increase of coconut shell addition(0wt%,10 wt%,25 wt% and 50 wt%)could reduce the contents of unstable acid soluble fraction(F1),reducible fraction(F2)and oxidizable fraction(F3)of heavy metals in sludg-based biochar.The stable residual fraction(F4)content increases,which effectively reduces the potential ecological risk level of heavy metals in sludge.However,when the amount of bentonite was 5 wt% and 10 wt%,the morphology of F1 and F2 heavy metals in sludge-based biochar increased,which increased the potential ecological and environmental risks of sludge-based biochar.With the increase of pyrolysis temperature and coconut shell addition,the H/C ratio and yield of sludge-based biochar decreased,while the specific surface area increased.FTIR characterization showed that the degree of aromatization of biochar was significantly improved.SEM and TEM characterization showed that the surface morphology of sludge-based biochar was rougher and pore development was more complete.The sludge-based biochar with bentonite showed opposite properties.Therefore,increasing the specific surface area,pore volume,roughness,lattice number and aroma of sludge-based biochar,and reducing the average pore size of adsorption of sludge-based biochar,all these changes promote the stabilization of heavy metals in sludge-based biochar and reduce the bioavailability of heavy metals.In this study,the adsorption process of coconut shell-sludge-based biochar for methylene blue,activated brilliant blue and ciprofloxacin conforms to quasi-secondorder kinetics and Langmuir monolayer adsorption model.At 25 °C,the equilibrium adsorption capacities for methylene blue,active brilliant blue and ciprofloxacin were6.97,14.84 and 43.10 mg/g,respectively.The adsorption process of sludge-based biochar for antibiotics is mainly chemical adsorption,and the adsorption mechanism include hydrogen bonding,π-π interaction and electrostatic interaction.It is proved that coco-husk-sludge-based biochar is an effective adsorbent for removing dyes and antibiotics from aqueous phase.